1. Field of The Invention
This invention relates to hydrogenated block copolymers of butadiene and isoprene. More specifically, this invention relates to such copolymers having at least five alternating blocks of the respective monomers.
2. Related Prior Art
Block copolymers of alpha-olefins have been produced by the periodic injection of several types of alpha-olefins resulting in block copolymers having properties somewhat different from random copolymers achieved by polymerization of mixtures of the two monomers. However, the products so obtained are either those of the thermoplastic type or of the elastomeric type requiring vulcanization to achieve their maximum stress strain properties.
Vulcanization of alpha-olefin polymers presents numerous problems which have not been satisfactorily solved at the present time. While vulcanization is possible, the vulcanization recipes are either elaborate or expensive or the products so produced have exceedingly bad odor, and other disadvantages are usually encountered.
It would be of special benefit to obtain an alphaolefin polymer having optimum elastomeric properties without the necessity of vulcanizing and at the same time a "self-vulcanizing" rubber which has a minimum of unsaturation, thus promoting its thermal and oxidative stability. Up to the present time, however, such products have not been obtained, either due to the lack of proper relationship of the individual alpha-olefin blocks within a given block copolymer, or to the solubility properties encountered during the polymerization process itself.
British Patent No. 1,160,234 describes the preparation of elastomeric block copolymers having the properties of a self-curing elastomer which has the general formula A--B--A wherein each polymer block A is an independently selected homopolymer block of an alpha-mono-olefin having 2 to 8 carbon atoms per molecule, each block A having an average molecular weight between 8,000 and 100,000 and preferably between 10,000 and 75,000, and B is an elastomeric copolymer block of at least two alpha-mono-olefins, having an average molecular weight between 35,000 and 500,000, and preferably between 40,000 and 350,000, the total block A content being between 10 and 50% by weight, preferably between 12 and 40% by weight of the block copolymer.
The patent describes the preparation of such saturated block copolymers from diene block copolymers having the structure C-D-C wherein each C represents a block of butadiene-1,3 homopolymer and D represents a homopolymer block of an alkyl-substituted conjugated diene such as isoprene. The C content is between 10 and 50% of the copolymer, and eventually at least 90% of the double bonds of the block copolymer are hydrogenated.
Specifically, a suitable starting block copolymer is described as polybutadiene-polyisoprene-polybutadiene. The block copolymer is then substantially completely (i.e. at least 90%) hydrogenated to provide essentially saturated block copolymers wherein the blocks A are not only saturated but are essentially non-elastomeric, while the block B is both saturated and elastomeric.
However, block copolymers having the three alternating blocks described above do not have satisfactory permanent set or low enough residual strain and upon milling do not give the desired smooth surface.